The rostral anteroventral cochlear nucleus (AVCN) of the chinchilla has provided a preparation in which neuronal cell bodies and synapses in the CNS can be examined after direct freezing and freeze-substitution of rapidly excised brain stem slices. The four types of synaptic terminals known to be in the AVCN were distinguished and correlated with four types of terminals previously reported after chemical fixation. Since the transmitters for each of the four types of terminals have been specified, the transmitter type could be correlated with the detailed structure of the postsynaptic density in each chemical type of synapse. Two types of filamentous components comprising the basic structure of the postsynaptic density differed specifically in each type of terminal, thus providing new information about structural differences between receptors arrays and associated cytoskeletal components at different types of central nervous system synapses. As part of a continuing effort to understand the basis of synaptic secretion by exocytosis, the formation of membrane pores induced by electroporation was studied with rapid freeze techniques. This approach provided detailed information on a millisecond time scale regarding the opening of membrane pores of our initial test preparation, human erythrocytes. A surprising finding was that the pores continued to open over the first tens of milliseconds before reaching a maximum size, suggesting that membrane interactions with the cytoskeleton were an important factor in pore formation. A new initiative uses confocal microscopy of organotypic brain slice cultures to study dynamic changes in structure during normal function and during development; reliable cultures and detailed imaging by confocal (fluorescent) microscopy have now been achieved.